Remove explicit create for pgpsql extension

This is now unneeded since we're on >=PG11 now.

db/structure.sql

 SET search_path=public;
 
-CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;
-
 CREATE EXTENSION IF NOT EXISTS pg_trgm WITH SCHEMA public;
 
 CREATE TABLE public.abuse_reports (

lib/gitlab/database/schema_cleaner.rb

@@ -12,27 +12,15 @@ module Gitlab
       def clean(io)
         structure = original_schema.dup
 
-        # Postgres compat fix for PG 9.6 (which doesn't support (AS datatype) syntax for sequences)
-        structure.gsub!(/CREATE SEQUENCE [^.]+\.\S+\n(\s+AS integer\n)/) { |m| m.gsub(Regexp.last_match[1], '') }
-
-        # Also a PG 9.6 compatibility fix, see below.
-        structure.gsub!(/^CREATE EXTENSION IF NOT EXISTS plpgsql.*/, '')
-        structure.gsub!(/^COMMENT ON EXTENSION.*/, '')
-
         # Remove noise
+        structure.gsub!(/^COMMENT ON EXTENSION.*/, '')
         structure.gsub!(/^SET.+/, '')
         structure.gsub!(/^SELECT pg_catalog\.set_config\('search_path'.+/, '')
         structure.gsub!(/^--.*/, "\n")
-        structure.gsub!(/\n{3,}/, "\n\n")
 
-        io << "SET search_path=public;\n\n"
+        structure = "SET search_path=public;\n" + structure
 
-        # Adding plpgsql explicitly is again a compatibility fix for PG 9.6
-        # In more recent versions of pg_dump, the extension isn't explicitly dumped anymore.
-        # We use PG 9.6 still on CI and for schema checks - here this is still the case.
-        io << <<~SQL.strip
-          CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;
-        SQL
+        structure.gsub!(/\n{3,}/, "\n\n")
 
         io << structure
 

spec/fixtures/gitlab/database/structure_example.sql

@@ -48,7 +48,6 @@ CREATE TABLE public.abuse_reports (
 --
 
 CREATE SEQUENCE public.abuse_reports_id_seq
-    AS integer
     START WITH 1
     INCREMENT BY 1
     NO MINVALUE

spec/fixtures/gitlab/database/structure_example_cleaned.sql

 SET search_path=public;
 
-CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;
-
 CREATE EXTENSION IF NOT EXISTS pg_trgm WITH SCHEMA public;
 
 CREATE TABLE public.abuse_reports (

spec/lib/gitlab/database/schema_cleaner_spec.rb

@@ -15,10 +15,6 @@ describe Gitlab::Database::SchemaCleaner do
     expect(subject).not_to include('COMMENT ON EXTENSION')
   end
 
-  it 'includes the plpgsql extension' do
-    expect(subject).to include('CREATE EXTENSION IF NOT EXISTS plpgsql WITH SCHEMA pg_catalog;')
-  end
-
   it 'sets the search_path' do
     expect(subject.split("\n").first).to eq('SET search_path=public;')
   end